docs: def _create_metadata_cache(cache_location):
      """Creates a new metadata cache instance appropriate for this platform.
  
      """
      cache_url = os.getenv('GUTENBERG_FUSEKI_URL')
      if cache_url:
          return FusekiMetadataCache(cache_location, cache_url)
  
      try:
          return SleepycatMetadataCache(cache_location)
      except InvalidCacheException:
          logging.warning('Unable to create cache based on BSD-DB. '
                          'Falling back to SQLite backend. '
                          'Performance may be degraded significantly.')
  
      return SqliteMetadataCache(cache_location)